The present invention relates in general to a digital timer employed in a combution control apparatus for controlling combustion process in a burner used in hot water heaters, hot air furnaces, or the like. In particular, the invention concerns a circuit for checking the timer in respect of operation at the beginning of the operation of the combustion apparatus.
In general, the combustion control apparatus is provided with two types of timers. One timer is used for controls. For example, the timer is used for setting a period of time in during which the air in the combustion chamber is purged prior to the fuel supply or setting a predetermined period of time during which the igniting device is continued to operate. The other timer is a so-called safety timer which is destined for stopping the operation of the combustion apparatus in the case where firing does not take place during a predetermined period of time after the initiation of the igniting operation. More particularly, the safety timer inhibits the injection of fuel upon failure of ignition thereby to stop the operation of the combustion apparatus.
These timers are usually constituted by flip-flop circuits. Assume now the case where failure or malfunction occurs in these timers so that frequency dividing operation can not be effected. In particular, there arises a very serious situation in the case of failure or mal-function of the safety timer, as will be appreciated from the function imposed on the safety timer described above. For example, assuming that the timing operation of the safety timer becomes ineffective and the time preset by the timer increases indefinitely, the ignition failure, if happens, will causes the fuel to be continuously discharged without combustion. On the other hand the failure of the control timer will give rise to instantaneous firing at the initiation of operation without the prepuriging process being interposed, possibly involving explosion due to the ignition of the remaining combustible gas.